We’re in peak migration season for birds, and that means you’re probably seeing unusual numbers of common yellowthroats.
These birds (Geothlypis trichas) are one of the most common warblers in North America (hence the name). They begin arriving in Massachusetts during the first week of May and begin migrating south in September. Unlike some species of birds, they do not migrate all at once. The period from mid-September to mid-October is their peak season, and some will still be headed south in November. Songbirds such as the yellowthroat may double their body mass before they migrate. Most of this increase is fat. Unlike people, who burn carbohydrates to provide energy, birds burn fat. The farther birds migrate, the more fat they need to store.
In addition, some migrating yellowthroats stopover for days or weeks along the way to refuel and rest. We see yellowthroats most often during their migrations because many pass through our region. The earliest fall migrants from the north reach Florida in the middle of September. In late September, they reach Cuba, and by October, some have reached Jamaica.
Unfortunately, common yellowthroats have one of the heaviest losses of life of any bird during their spring and fall migrations. In 1953, the ornithologist and Taunton native Arthur Cleveland Bent wrote that “The number of yellowthroats that fall victims to natural enemies are insignificant when compared with the appalling losses suffered by this species during the migration.” Buildings mixed with bad weather present a particular problem – in Philadelphia in 1915, 130 mostly female yellowthroats died by flying into buildings and the new City Hall with its electric lighting. A heavy fog followed by rain made the birds fly lower than usual.
Large buildings with a lot of glass cause the most carnage, but building height does not affect the number of bird collisions as much as the amount of light projected – the more light a building emits, the greater the number of birds that collide with it. Residential homes kill about 253 million birds per year.
Birds collide with buildings during the daytime in the lowest 250 feet of the building. These birds mistake reflections in glass for blue skies or vegetation. Common yellowthroats, for example, fly at high speeds hunting for insects, and may mistake the scene reflected in the glass as a safe passage.
At night, birds use celestial clues to guide their travels. Light emitted from buildings can disorient them, diverting migrating birds away from their normal path. These birds can collide with windows, or become so disoriented or exhausted that predators catch them. Migratory, insect-eating birds such as the common yellowthroat are more likely to collide with glass than other types of birds.
Being disoriented can make thing worse for common yellowthroats than for other birds. Common yellowthroats are social songbirds that chirp during nighttime migration, known as flight calls. Flight calls help the birds make collective decisions, such as where to fly. But this same collective behavior can lead to their deaths. When these birds lose their bearings because of artificial light, they chirp, which lures in other birds. These other birds also become disoriented and eventually collide with the light-emitting windows.
The fall migration kills more birds than the spring migration. This may occur because recently hatched, inexperienced birds fly south in the fall along with their more experienced parents. In addition, birds take longer routes to fly south in the fall than they do to fly north in the spring. This increased travel time presents more chances for storms or other dangers to befall the migrants.
There are 3 billion fewer birds in North America than there were in 1970. Most of the birds that have been lost (2.5 billion) are migratory birds.
The good news is, we can all help reduce bird deaths from collisions. New York City’s Javits Center, designed by famed architect I.M. Pei, used highly reflective glass that made it one of the worst buildings in New York City for bird fatalities. When the building was renovated in 2014, the highly reflective glass was replaced with bird-friendly glass that was less than half as reflective and has a dot pattern that blurs reflections. This new glass has reduced bird collisions by 90 percent (and has lowered the building’s energy costs by more than 25 percent). To help migrating birds, the center turns off nonessential lights at night.
We can do the same in our homes, helping migrating birds by turning off unnecessary lights at night, closing curtains or blinds, installing motion sensor lights to ensure that lights come on only when needed and making sure that outdoor lights point downward.
If you’d like to learn more about how birds are affected by lights and glass in cities, the National Audubon Society in Philadelphia monitors this situation and has learned a great deal about what people can do to protect them.
Next week I will write more about the amazing and mysterious phenomenon of migration.
A stronger
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